Add Date/DateTime.dd-mm-yyyy / mm-dd-yyyy methods
Add Date/DateTime.dd-mm-yyyy / mm-dd-yyyy methods

Seeing the code from https://covid.observer made me realize that
most uses of the formatter for Date/DateTime is to create the
mm-dd-yyyy and dd-mm-yyyy alternatives for dates.  Since these
methods would be 250x faster than using a sprintf and the formatter,
it seems like a good thing to add, especially since these methods
allow for a different separator so today would can show up for our
American friends as:

    say Date.today.mm-dd-yyyy("/");  # 03/14/2020

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